1. Field of the Invention
The invention relates to a method for transmitting synchronization messages in a communications network and a node in a communications network and a corresponding communications network.
2. Description of the Related Art
In many technical fields, communications networks are used to perform distributed processes automatically on a plurality of units. Particularly in industrial automation networks, it is very important that the automatic processes are precisely matched to one another. To this end, the individual nodes in the communications network comprise suitable clocks that are synchronized to a reference clock in a reference node. The reference node is also often known as the master node, whereas the other nodes with internal clocks are designated slave nodes.
Protocols known from the prior art for synchronizing the clocks in a communications network are defined Institute of Electrical and Electronics Engineers (IEEE) industrial standard 1588, 1588v2 and 802.1AS, and International Electrotechnical Commission (IEC) standard 61158. According to these protocols, synchronization messages are exchanged as packets having time stamps. A synchronization message issued by the master node is passed on successively between the slave nodes. The synchronization message issued by the master node contains a time stamp of the first clock count state according to the reference clock at the time of issuing of the message. The slave nodes process this information and issue further synchronization messages. Each slave node adds the estimated delays between the issue time point of the synchronization message in the previous node and the issuing time of the node in question at the first clock count state in the received synchronization message. The clock count state resulting therefrom is inserted into the synchronization message to be issued. A slave node can possibly also store the first clock count state from the synchronization message issued by the master node in a first field and add the estimated delays in a separate second field. Based on the information in the received synchronization messages, each slave node can synchronize the second clock count states thereof according to the internal clock thereof to the first clock count states thereof according to the reference clock.
The internal clocks in the slave nodes usually do not exactly match the reference clock of the master node. As a result, the ratio of the reference clock frequency to the internal clock frequency is estimated in each slave node. This ratio, which is also designated the rate compensation factor (RCF) value, is used to stipulate relevant compensation intervals. For the measurement of a delay in the respective slave, based on the internal clock thereof, the second clock count state thereof is always changed when the measured delay exceeds the end point of a compensation interval. This method has the disadvantage that the delay times thereby determined only partially compensate for the deviations between the internal clock of the slave and the reference clock of the master. For example, no compensation of the delay time occur at all if the delay time lies within a compensation interval. There is also significantly too much compensation if a very short delay time exceeds the end point of a compensation interval.